In modern mobile communication appliances, which are designed for a simultaneous transmission and reception mode, the problem may arise that a signal for transmission is injected into the reception path from the transmission path. An injected signal increases the probability of error when the received signal is demodulated, or even prevents reception entirely. This problem becomes greater, the greater the level differences are between a received signal and the transmission signal. If the mobile station is at a very great distance from a base station, for example, transmitted signals need to be radiated from the mobile appliance to the base station at relatively high power. At the same time, however, the signals which are transmitted by the base station are received only at a very low power, which means that a large power or level difference is obtained. The example of the mobile radio standard UMTS is intended to be used to explain this problem in more detail.
In the specification TS25.101, the UMTS standard defines the frequency ranges and various power classes which are characterized by a maximum output power. By way of example, a transmitter for UMTS signals in the mobile communication appliance is able to radiate a signal at an output power of 24 dBm, and at the same time the receiver in the mobile communication appliance can receive a signal at −90 dBm. The interval between transmission frequency and reception frequency is 190 MHz. To prevent injection at this frequency interval on account of the large level differences between the transmitted signal and the received signal, conventional transceivers use a duplexer. This is usually formed with a circulator and routes the transmitted signal sent by the transmission path to the antenna, where it rejects the transmitted signal in the direction of the reception path. In conventional duplexers, the rejection is approximately 45 dB, which in the example cited above still results in the transmitted signal being injected into the reception path at a level of −21 dBm.
To reduce the level of the crosstalking signal even more, further measures are accordingly required. In the embodiment which is shown in prior art FIG. 4 and which is known to the applicant, narrowband SAW filters 88 are additionally used. These are connected both to the transmission path 2 and upstream of the reception path 3. In particular, an SAW filter 88 is connected downstream of a reception amplifier 33a and upstream of the input of the reception path 3.
The SAW filter connected to the transmission path is used for rejecting any image frequencies there may be, on account of the mixing process in the transmission path and for rejecting intermodulation products. The narrowband SAW filter, which is connected to the output of a reception amplifier 33a, has a typical attenuation of approximately 25 to 30 dB and thus rejects the remaining crosstalking signal to a level of −45 to −51 dBm. The amplifier 33a is needed in order to compensate for the additional attenuation of the SAW filter 88 within the useful signal band and to decouple the duplexer 7a from the SAW filter 88. This means that the signal-to-noise ratio for the received signal is improved in the useful signal band. The narrowband SAW filters used are very expensive to produce in the frequency range of approximately 2 GHz which is used, however. This increases both the cost and the space taken up by a transceiver for UMTS signals.
Another possibility is demonstrated by the printed document DE 199 34 502. In this document, a compensating element is connected in parallel with the antenna to the output of a transmission amplifier and upstream of the input of the reception amplifier. Appropriate control of the compensating element allows a signal to be produced which is additively superimposed on a received signal and thus reduces a transmitted signal component on account of crosstalk from the transmission path into the reception path. This embodiment allows the SAW filters to be saved and the costs to be reduced.